I bought this amp knowing it needed work. There is a fair amount of ground noise or hum (not sure if it is a ground issue) when something is plugged into the inputs.

If I don't have anything plugged in and turn up the channel volume and master volume all the way, the noise is way less.

I rewired the inputs in the schematic below because the wires and resistors from the stocking wiring broke, I'm just wondering if I wired it right.

The input jacks are TRS.

Looks like, give the schematic, that the HIGH input is supposed to have the TIP going to one end of one of the 68k resistors, with the RING and SLEEVE of the HIGH IN going to ground.

The Low Input TIP goes to one of the other 68K resistors and the RING going to the TIP of the HIGH In, and the SLEEVE going to ground.

Is that correct? I'm wondering if I wired it wrong and that is why I'm getting this hum.


*** Please note there is a cap that is highlighted in the linked picture, ignore that**

Thanks.

 

The signal goes on the tips. Everything else is ground.

While you're ignoring the caps, they are the most likely source of hum.
This model also has no safety ground so reversing the power plug might reduce hum, and it might not. The input power is supposed to be completely isolated from the case. Be sure it is or it might knock you on your caboose.

Chasing a hum problem in these antiques can be frustrating, even for an experienced repairman. Someone that isn't sure about how to wire the input plugs? Magic eight ball says, "outcome uncertain".

 

The signal goes on the tips. Everything else is ground.

While you're ignoring the caps, they are the most likely source of hum.
This model also has no safety ground so reversing the power plug might reduce hum, and it might not. The input power is supposed to be completely isolated from the case. Be sure it is or it might knock you on your caboose.

Chasing a hum problem in these antiques can be frustrating, even for an experienced repairman. Someone that isn't sure about how to wire the input plugs? Magic eight ball says, "outcome uncertain".

I've changed all the Electro caps except the three 333uf 500v caps..

Also, someone added a three prong chord. The attached the ground (green) to one of the negative leads of one of those 333uf caps.

should I put it someone else?

 

Yes. Attach the green ground to the metal case. The case will carry the ground to the input jacks and the input jacks will carry the ground to the circuit boards.

The green wire connection to the circuit board might be correct, or not. I'd have to study the schematic to decide. Meanwhile, try disconnecting the green from the circuit board and just connect it to the chassis. You might be causing a ground loop by attaching to the circuit board. No harm in trying. If it fixes nothing and harms nothing, the connection to the metal case is the right way to do it.

 

I hooked up the power cord green to the chassis ground - with a washer to one of the bolts of the power transformer. That didn't help. I'm wondering about this other connection here in the pic below. If you look at the schematic. There is a 1000uf 50v cap. There is a ground tap right next to it and the wire runs to a chassis tab and another wire comes from ground of the three 333uf caps.

Might this be an issue? pic below.

 

Grounding theory...there are so many ways to do it. Personally, I allow one (1) connection from the green wire to the case. I do not use the case for current grounds. For instance, the 1000 uf capacitor is the first filter for the positive voltage for all the circuit boards. I would hand wire from the negative side of that capacitor to each circuit board, unless that is already taken care of with circuit board traces.

The 500 volt caps supply the output stage. I would make sure there is a wire from the phase splitter and output tubes, right back to the negative end of the 500 volt caps...not to case ground. It's about current loops. You don't need current intentionally using the case to get back home. You need to serve the current loops back to their own sources so the output power doesn't get mixed with the input signal by trying to travel in the same conductor.

Now, I fully expect some self-professed expert to contradict me.

 

ok, I experimented before you replied and removed the ground from the 1000uf to the chassis and that helped.

So for the 500v output stage caps. I should take that remaining ground wire that runs to the chassis off?

The output of the preamp/ SS board has a shielded wire that goes the output/cap PCB. The ground from the shielded wire touches the out pcb ground....

 

ok, I experimented before you replied and removed the ground from the 1000uf to the chassis and that helped.

Yay. Sometimes following proper theory actually improves the product.

So for the 500v output stage caps. I should take that remaining ground wire that runs to the chassis off?

I would. Then I'd make sure a wire exists from the negative side of the 500 volt caps to the phase splitter and power output stages.

The output of the preamp/ SS board has a shielded wire that goes the output/cap PCB. The ground from the shielded wire touches the out pcb ground....

That one is proper.

It is my opinion that an amateur has been in there with the belief that more grounds are better. It ain't necessarily so. Too many grounds cause current loop problems and hum pickup. You need to install exactly the right number of grounds, for the right reasons, and dress your ground wires close to the case.

 

A couple of thoughts. Remember that 60Hz hum comes from the transformer. Watch for any wires that pass close to the transformer. I will assume that the manufacturer placed the mains transformer and the output transformer in their optimum locations. Can you show photos of both the top and bottom sides?

I would remove all grounds to the chassis except the one ground wire from the power cord.

Next, look at all the jacks and find out if they are grounded to the chassis. (If they are grounded you shouldn't need another ground.)

Finally, connect one ground from the circuit to the chassis. Finding the right connection will require some experimentation.

 

Pins 8 for both Power tubes are grounded to the chassis via solder tab on the chassis.

The jacks are grounded to the chassis. There is a shielded wire running from the input jacks to the circuit board, tying into a ground there (also on one of the jacks, think I should omit that shielded input?

 

In theory, shielded cables should be grounded only at one end. Experiment both ways and see what difference it makes.

 

The signal from the musical instrument gets a ground reference at the input jack. I prefer to use the shield of the cable to run from the jacks to the preamps. That keeps the guitar (or whatever) from the need to move its ground current through the chassis.

I'm thinking the one end only theory works for twisted, shielded pairs, but guitars aren't that sophisticated.

 

what about power tube pins that I mentioned were grounded to the chassis?

 

I would rewire those to run back to the power supply and not through the chassis.

 

Agreed. The power tubes provide the most offensive current through the chassis, but then, I covered this in post #8. Wire them directly to the negative end of the last 500 volt cap.

 

Hey y'all, I appreciate the help, totally fixed the noise.

The amp doesn't sound that great though - this isn't ground related, I don't think, but I wanted to know where I might start and try to trouble what may be causing the issue. I'm not sure if it is a design thing - that the amp just doesn't sound that great, or if there is something else wrong that is inhibiting the amp's performance.

There aren't any other sound issues occurring, amp is quiet, its just it sounds like it has a blanket over it tonally, the highs are not very present.

Any advice on where to probe and or mod suggestions?

 

I wouldn't even try without a signal generator and an oscilloscope. In fact, I spent over a thousand dollars to buy those two tools to do this exact job. If I would spend over a thousand dollars for the equipment, do you really think I could guess how to insure the proper audio spectrum through your amplifier from here?

Try a different speaker. Other than that, you gotta be kidding.

 

Could you provide the part numbers for IC1 and IC2?
Do you know how to read the schematic and locate solder points on the circuit board?
If not, you will have to post clear, well focused photos of the top and bottom of the circuit board.

 

HD14016...It's on the schematic in the upper left.

So, you're going to fix a subjective measurement over the internet?
WOW!

 

Piece of cake!